1. Field of the Invention
This invention relates generally to telecommunications, and, more particularly, to a multi-tone transceiver for multiple users.
2. Description of the Related Art
In communications systems, particularly telephony, it is common practice to transmit signals between a subscriber station and a central switching office via a two-wire bi-directional communication channel. A line card generally connects the subscriber station to the central switching office. The primary functions of the line card range from supplying talk battery to performing impedance matching to handling ringing signal, voice and data signals, and testing signals. Until recently, line cards generally supported only the Plain Old Telephone System (POTS). However, a recent trend has been to utilize line cards to support protocols for transmission of digital data.
The Plain Old Telephone System, designed primarily for voice communication, provides an inadequate data transmission rate for many modem applications. To meet the demand for high-speed communication, designers sought innovative and cost-effective solutions that would take advantage of the existing network infrastructure. Several technological advancements were proposed in the telecommunications industry that made use of the existing network of telephone wires. The most promising of these technologies is the xDSL technology.
DSL is making the existing network of telephone lines more robust and versatile. Once considered virtually unusable for broadband communications, an ordinary twisted pair equipped with DSL interfaces can transmit videos, television, and very high-speed data. The fact that more than six hundred million telephone lines exist around the world is a compelling reason that these lines will serve as the primary transmission conduits for at least several more decades. Because DSL utilizes telephone wiring already installed in virtually every home and business in the world, it has been embraced by many as one of the more promising and viable options.
DSL technologies leave Plain Old Telephone Service undisturbed. Traditional analog voice band interfaces use the same frequency band, 300 Hertz (Hz)-4 Kilohertz (KHz), as telephone service, thereby preventing concurrent voice and data use. A DSL interface, on the other hand, operates at frequencies above the voice channels from about 30 KHz to 1.1 Megahertz (MHz). Thus, a single DSL line is capable of offering simultaneous channels for voice and data.
DSL systems use digital signal processing (DSP) to increase throughput and signal quality through common copper telephone wire. It provides a downstream data transfer rate from the DSL Point-of-Presence (POP) to the subscriber location at speeds of up to 6 Mega-bits per second (MBPS). Even a more modest transfer rate of 1.5 MBPS, for instance, is fifty times faster than a conventional 28.8 kilobits per second (KBPS).
One popular version of the DSL technology is the Asymmetrical Digital Subscriber Line (ADSL) technology. The ADSL standard is described in ANSI T1.413 Issue 2, entitled, xe2x80x9cInterface Between Networks and Customer Installationxe2x80x94Asymmetric Digital Subscriber Line (ADSL) Metallic Interface, Rev. R6, dated Sep. 26, 1997, incorporated herein by reference in its entirety.
ADSL modems use two competing modulation schemes: discrete multi-tone (DMT) and carrierless amplitude/phase modulation (CAP). DMT is the standard adopted by the American National Standards Institute.
The technology employed by DMT ADSL modems is termed discrete multi-tone. The standard defines 256 discrete tones. Each tone represents a carrier signal that can be modulated with a digital signal for transmitting data. The specific frequency for a given tone is 4.3125 KHz times the tone number. Tones 1-7 are reserved for voice band and guard band (i.e., tone 1 is the voice band and tones 2-7 are guard bands). Data is not transmitted near the voice band to allow for simultaneous voice and data transmission on a single line. The guard band helps isolate the voice band from the ADSL data bands. Typically, a splitter may be used to isolate any voice band signal from the data tones. Tones 8-32 are used to transmit data upstream (i.e., from the user), and tones 33-256 are used to transmit data downstream (i.e., to the user). Alternatively, all the data tones 8-256 may be used for downstream data, and upstream data present on tones 8-32 would be detected using echo cancellation. Because more tones are used for downstream communication than for upstream communication, the transfer is said to be asymmetric.
Through a training procedure, the modems on both sides of the connection sense and analyze which tones are less affected by impairments in the telephone line. Each tone that is accepted is used to carry information. Accordingly, the maximum capacity is set by the quality of the telephone connection. The maximum data rate defined by the ADSL specification, assuming all tones are used, is about 8 MBPS downstream and about 640 KBPS upstream.
A typical point-to-point ADSL connection uses an ADSL transceiver at the central office (CO) connected to another ADSL transceiver via a subscriber line. At the CO, a plurality of transceivers is located on a line card. However, the number of transceivers that can be placed on one line card is limited because of space constraints and power dissipation concerns. As a result, the ADSL service is available to a relatively small fraction of all potential users. Additionally, the processors of the ADSL transceivers are not always efficiently utilized, particularly between downloads, where the ADSL processors are essentially idling. Thus, the inefficient use of the ADSL processors limits the number of users that can have access to a multi-tone transceiver at any given time.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.
In one aspect of the present invention, a method is provided for supporting a plurality of user transceivers with a host transceiver. The method includes allocating at least one symbol of a DMT frame to a first user transceiver of the plurality of transceivers, providing a control signal from a second user transceiver of the plurality of transceivers to the host transceiver, allocating at least one symbol of the DMT frame to the second user transceiver in response to the control signal.
In one aspect of the present invention, an apparatus is provided. The apparatus includes a transmit block capable of transmitting data within a first portion of a DMT frame to a first user transceiver over a first connection. The apparatus includes a control block adapted to receive a control signal over a second connection, the control block capable of allocating a second portion of the DMT frame to a second user transceiver for data transmission in response to the control signal.